Be aware that there are two types of fiber out there - digestible fiber and indigestible fiber. Digestible fiber comes from the fiber in and attached to starches and sugars, mostly. Think the fiber in whole-wheat bread and brown rices. You do NOT want digestible fiber if you are looking to increase your fiber intake. I bring this up so when you see 'fiber' on a loaf of bread, you'll know it's not the fiber you want.
What you're looking for is more indigestible fiber - cellulose and lignin - to increase your gut mobility and water retention. The best foods for these are green or fibrous vegetables.
You may have just gone ugh! in your mind, but there are a lot of people who initially have that reaction that end up loving their veggies once they realize what's out there. The salads you're most familiar with are overwhelmingly likely to have paper-like lettuce beneath twice-frozen carrots, preserved cherry tomatoes, and other such poor-quality ingredients. If you get yourself some fresh, organic baby spinach and toss it up with sliced strawberries, chopped almonds or cashews, and dust it lightly with feta crumbles, your tune toward a salad could change very rapidly indeed.
You can check out this page to give you some idea of where to start, but if you don't see something you want right away, just google for 'high-fiber vegetables' and you'll get all sorts of information.